1. Field of the Invention
The invention relates to an electronic circuit for generating a transmit frequency for a transceiver.
2. Description of the Related Art
The inventors are familiar with similar circuits from the prior art for generating corresponding transmit frequencies in a TDMA radio system (for example DECT, GSM, PHS). The abbreviation TDMA stands for “Time Division Multiple Access”. A typical circuit is composed of an oscillator for generating frequencies, a transmit amplifier, a receiver and a control device which determines the chronological sequence of alternating transmit and receive states. In general, the oscillator frequency for setting the transmission channel via the control device using a PLL (phase locked loop) is set before the switching on of the transmitter since, for technical reasons, a certain setting time is required for this process. The invention relates to the case of transmission in such a TDMA system as illustrated schematically in FIG. 1.
The problem of such a simple circuit is that the generation of frequencies is disrupted at the moment of the switching on of the transmit amplifier owing to the load change in the amplifier or due to feedback. As a result, an undesired frequency jump is generated. Such a load change occurs, for example, during the switching on of the transmit amplifier as a result of the change in its input impedance. An effect on the generation of frequencies can arise, for example, owing to irradiation by the antenna, or due to other coupling parts between the transmit output stage and the generation of frequencies, for example due to the supply voltage.
In particular in TDMA systems which, for costs reasons, operate with a slow PLL control loop, or open the control loop for the duration of the modulation, this effect is a large problem for the implementation because the frequency jump can no longer be corrected by the PLL circuit. An example of this is the open-loop modulation of a DECT system.
The abovementioned problem is tackled by various circuits known to the inventors. For example, there is a possibility of bringing about a reduction in the load change which is visible for the generation of frequencies by inserting damping elements and isolating stages between the frequency generating components and the transmit amplifier. In addition, additional shielding of the frequency generating components in the form of a Faraday cage can ensure that the irradiation is reduced. Furthermore, additional blocking against electromagnetic irradiation, for example by specially shaped plugs, can be provided on the lines which lead into the shield. An example of such a known circuit device is shown in FIG. 2.
It is also known that the insertion of frequency multiplication stages or divider stages in the frequency generating components prevents the feedback and thus the influence on the frequency generating components. Here, an oscillator oscillates at a harmonic or subharmonic of the desired frequency, as a result of which both a low load dependence and a lower sensitivity to the irradiation of undesired frequencies is produced in accordance with the degree of multiplication or division. This circuit is illustrated schematically in FIG. 3.
Finally, the relatively costly use of a transmission mixing concept, such as is illustrated schematically in FIG. 4, for solving the abovementioned problem is known to the inventors.
In this transmission mixing concept, the frequencies of two oscillators are mixed in a mixer stage and the desired frequency filtered out from the mixing products. Because the oscillators have a nonharmonic relationship with the desired frequency, there is a resulting high degree of immunity to the load changes and effects. As a result, the requirements made of the shielding, the blocking and the isolation stages are reduced considerably in comparison with the known solutions from FIGS. 2 and 3.
The greatest disadvantage of this transmission mixing concept is the large degree of technical expenditure which it requires because a transmission mixer stage, an oscillator including a PLL circuit for frequency stabilization and a band filter are additionally required. The additionally required electronic components alone result in a considerable cost disadvantage in comparison with the two preceding solutions.
A further disadvantage of this more costly transmission mixing concept is that the overall size of such a circuit is too large owing to the number of additional electronic components.
In this transmission mixing concept, it proves particularly difficult to achieve a high degree of integration because given the current state of the art the filters and oscillators or oscillator coils are very difficult to accommodate in integrated circuits, or require a very large chip area. In addition, it is frequently impossible to integrate to a sufficient degree the capacitors and resistors which are required for the PLL so that they have to be arranged as external components.
Because a total of two oscillators for frequency stabilization, two PLLs, including two external loop filters, are necessary in the known transmission mixing concept, and in particular oscillators with a low frequency require a particularly large chip area or have poor properties with respect to phase noise, this transmission mixing concept proves relatively unsuitable for a high integration density.